A CDMA 1X/EVDO hybrid access terminal (mobile device) is a hybrid system that has evolved from the CDMA 2000 system. EVDO standards for Evolution Data Only or Data Optimized and, as suggested by this, is a data only system. Alternatively, EVDO is also known as High Rate Packet Data (HRPD). One advantage of EVDO systems is that they allow a higher transfer rate for data. They are also useful to carriers to clear the 1X system for more voice capacity by removing data traffic from the 1X system. The 1X system is also known in the art as 3G1X.
A hybrid access terminal operating in a 1X/EVDO hybrid mode will first acquire the CDMA 1X system and once this system has been acquired and the hybrid access terminal is in an idle stage, it will try to acquire an EVDO system. The CDMA 1X system will always have precedence over the EVDO system according to current design specification in terms of initial system acquisition. Once the 1x system is acquired, a hybrid access terminal starts periodic monitoring for the availability of any EV-DO systems as EVDO system is more preferred for packet data service and the fact that EVDO system also provides the capability to receive an incoming voice call even during active packet data session which is not possible on 1x system.
On transition from a CDMA 1X system to an EVDO system, or vice versa, a hybrid access terminal with a dormant packet data session initiates a data call to the new system. This is required so that the device connects the current data session with the new system upon transition as the wireless network may need to move the data context information associated with the hybrid access terminal from source to target network. This is also known as packet data “reconnect” process. This occurs when the device transitions from the EVDO system to the 1X system, or vice versa.
If for any reason the transition between and EVDO system and a 1X system fails, a problem exists with redundant data calls being made. For example, a problem exists in boundary areas where the EVDO system may be seen by the hybrid access terminal and the terminal attempts to make a transition due to preference of EVDO system over 1x system. During such transition attempts, EVDO system may not be able to be acquired, or may be acquired and quickly dropped due to signal strength variations or the hybrid access terminal moving into and out of the boundary area. The problem with this is that a data call is made if the hybrid access terminal fails to transition to the EVDO system and moves back to a 1X system. Since the network considers the device in the 1X system already, this data call is redundant and wastes mobile battery life and network resources.